4.1 Total Suspended Particulates (TSP)
Total suspended particulates (TSP) are small airborne particles such as dust, fume and smoke with diameters less than 100 micrometres. They are emitted from various sources including power stations, construction activities, incineration and vehicles.
The fine portion, which are known as the respirable suspended particulates below, are of greater health concern. The coarse particles are mainly related to soiling and dust nuisance.
Levels of TSP in 1996 are depicted in Figure 6. The concentrations remained high throughout the territory although a slight decrease in the overall concentration was noted when compared with the figures of 1995. Six out of the nine monitoring stations recorded violation of the annual air quality objective. The highest annual level at the street site of Mong Kok was almost 78% above the limit. Due to closer proximity to emissions from vehicles and surrounding construction activities, it also recorded two violations of the 24-hour AQO limit. Same as previous years, Sha Tin recorded the lowest annual level which was about 86% of the permissible limit.
4.2 Respirable Suspended Particulates (RSP)
Respirable suspended particulates (RSP) are airborne particles with diameters of 10 micrometres or less. Apart from combustion sources, in particular diesel vehicles, atmospheric oxidation of sulphur dioxide and nitrogen oxides and to a less extent, the crustal dust and marine aerosols are also a significant sources of RSP.
RSP may cause chronic and acute effects on human health, particularly the pulmonary function, when exposed at high level as they can penetrate deep into the lungs and cause respiratory problems. These effects are enhanced if high RSP levels are associated with higher levels of other pollutants, such as SO2. Smaller particles in RSP will also have a major impact on visibility.
The RSP concentrations recorded in 1996 at various station are shown in Figure 7. There was one violation of the 24-hour AQO limits at Kwun Tong. The annual RSP levels were also high. Four sites, viz., Kwun Tong, Yuen Long, Sham Shui Po and Mong Kok, violated the annual AQO for RSP in 1996. The highest annual level at the street site of Mong Kok was almost 40% above the limit. Diesel vehicle emissions were the major cause of the high RSP concentrations. Same as TSP, Sha Tin recorded the lowest annual level of about 84% of the permissible limit.
Lead in airborne dust mainly comes from combustion of leaded petrol. Due to the reduction of lead in petrol programme, the ambient lead concentrations remained very low in 1996. Higher lead content was found at the Yuen Long station. It was possibly caused by high traffic flow and industrial activities in the surrounding area.